1. Field of the Invention
The present invention relates to a method and apparatus for producing and detecting an image of a topographic surface, such as a fingerprint, for optical viewing or storage in a recording medium such as photographic film, a hologram, or computer storage means. Such topographic images may be analyzed or compared with previously recorded topographic data. Such methods and apparatus are used, for example, for personal identification in the criminology and access control fields.
2. Description of the Related Art
Examination of the fingerprints or other biometric features of an individual is a well known technique to establish a person's identity. In the past, fingerprints were recorded by applying ink to the finger and then rolling the finger on a piece of paper. Agencies then maintained these originals, or photocopies or photographs of the originals. Later comparison of these archives with a live or latent fingerprint could prove to be quite tedious and time consuming. More recently, methods have arisen to create fingerprint images which can be captured directly electronically for storage and/or analysis by a computer, or photographically for storage on film.
Many methods have been proposed for optical detection of fingerprints and other topographic biometric features such as footprints and palmprints. Most of these methods utilize optical systems which typically incorporate a prism or beamsplitting apparatus, and operate on the principle of frustrated total internal reflection. Such systems are necessarily bulky, cannot detect certain skin detail information, such as valley or pore detail, or require expensive optical components. In addition some prior art systems suffer from distortion, poor signal to noise ratio, aberrations, lack of contrast, and/or lack of resolution of finger detail information.
There are many patents which describe optical systems which use prisms to produce illuminating light. In such systems, one side of a prism is illuminated with light, while a subject's finger is pressed against another surface of the prism, usually its hypotenuse. A fingerprint image is formed by frustrated total internal reflection where the finger meets the prism. Light passing out of the prism containing the fingerprint image is then captured by an image detection system. Examples of prism-based finger print detection systems can be found in U.S. Pat. No. 3,174,414 to Myer, U.S. Pat. No. 3,482,498 to L. Becker and U.S. Pat. No. 3,947,128 to Z. Weinberger, et al.
A number of prior art systems have been developed, which illuminate the finger directly without using the principle of total internal reflection. U.S. Pat. No. 5,177,802 to Y. Fujimoto, et al. describes several finger print detection systems which do not use the principle of frustrated total internal reflection. As disclosed therein, one such system uses a light guiding plate having a through hole, so that the finger is in air, and light traveling through the light guide exits the light guide near the hole to illuminate the finger. The finger image is captured by a detection system directly opposite the finger. In another system disclosed in this U.S. Patent, a light guiding substrate is not used, and instead, a pair of linear light sources, such as fluorescent tubes, are placed near the `belly` of the finger to be imaged, and detection system images the finger directly. This embodiment suffers from bulkiness and direct lamp light reaching the image detector, thus reducing the signal to noise ratio of images produced using this system. In other embodiments, the finger is illuminated directly via light emerging from the light guiding plate. However, in such systems, the light strikes the finger at an angle, limiting uniformity of illumination and contrast.
Various fingerprint detector systems have been developed, using image comparison and illumination scanning techniques. For example, U.S. Pat. No. 3,511,571 describes a system which floods the surface of the finger with light. The light reflected from the finger is passed through a transparency of a previously recorded fingerprint, for direct comparison. The systems disclosed in U.S. Pat. Nos. 3,200,701; 3,864,042; and 4,003,656 scan a light beam across a finger and detect light therefrom in order to produce an output signal for constructing a fingerprint image.
U.S. Pat. No. 3,430,300 to H. Ruell describes a system for forming a latent topographic relief of a finger pattern. This system uses a deformable polymer which is illuminated through its edge, while a finger is pressed thereupon. The deformation of the polymer, due to finger pressure, frustrates total internal reflection of the light, thereby redirecting the illuminating light at the points of frustration to the output face of the substrate.
Various methods have been proposed utilizing holograms as part of the fingerprint illumination and detection system. U.S. Pat. No. 5,109,427 describes a fingerprint recognition device which uses collimated laser light to illuminate a finger through a tetragonal prism. A hologram is used to shift the axis of the fingerprint image to a second optical axis, so that an objective lens can then project the fingerprint image to a CCD camera.
The paper entitled "Real-time fingerprint sensor using a hologram", Applied Optics, Vol. 31, No. 11, p.1794ff, and U.S. Pat. No. 4,728,186 entitled `Uneven Surface Date Detection Apparatus`, each describe a system which uses laser light to illuminate a finger directly using a light guiding substrate. The scattered light reflected from the finger travels through the light guiding substrate and is permitted to exit therefrom through a plain grating-type hologram attached to the substrate. A lens aligned with the plain guiding-type hologram then forms an image of the fingerprint which is subsequently detected by a CCD camera or other detector. U.S. Pat. No. 4,728,186 also discloses a system which uses a hologram to direct the light to the finger. However, in this prior art system, the image travels by total internal reflection towards a hologram which directs the image towards an image detection system located along a different optical axis from the finger. As the input light path to the finger and the output light path to the detection system are necessarily along different optical axes, this prior art system is likely to suffer from (i) loss of information due to imperfections in the waveguide-type light guiding substrate, and/or (ii) damage to the image information (e.g. addition of noise, distortions or aberrations) due to propagation of light rays through the waveguide and/or the light extracting hologram. In addition, the geometry of this prior art system limits its ability to be extremely compact.
Other prior art systems make use of holograms, but for different purposes. For example, in U.S. Pat. No. 4,053,228, a fingerprint is compared against a fingerprint which is stored on a hologram.
Thus, there is a great need in the art for an improved method and apparatus for producing high-contrast images of the surface topography of objects, such as finger and foot surfaces, while avoiding the shortcomings and drawbacks of prior art systems and methodologies.